public class WindowCache extends Object
PackFile in memory for faster read access.
The WindowCache serves as a Java based "buffer cache", loading segments of a PackFile into the JVM heap prior to use. As JGit often wants to do reads of only tiny slices of a file, the WindowCache tries to smooth out these tiny reads into larger block-sized IO operations.
Whenever a cache miss occurs, load(PackFile, long) is invoked by
exactly one thread for the given (PackFile,position) key tuple.
This is ensured by an array of locks, with the tuple hashed to a lock
instance.
During a miss, older entries are evicted from the cache so long as
isFull() returns true.
Its too expensive during object access to be 100% accurate with a least recently used (LRU) algorithm. Strictly ordering every read is a lot of overhead that typically doesn't yield a corresponding benefit to the application.
This cache implements a loose LRU policy by randomly picking a window comprised of roughly 10% of the cache, and evicting the oldest accessed entry within that window.
Entities created by the cache are held under SoftReferences, permitting the Java runtime's garbage collector to evict entries when heap memory gets low. Most JREs implement a loose least recently used algorithm for this eviction.
The internal hash table does not expand at runtime, instead it is fixed in size at cache creation time. The internal lock table used to gate load invocations is also fixed in size.
The key tuple is passed through to methods as a pair of parameters rather than as a single Object, thus reducing the transient memory allocations of callers. It is more efficient to avoid the allocation, as we can't be 100% sure that a JIT would be able to stack-allocate a key tuple.
This cache has an implementation rule such that:
load(PackFile, long) is invoked by at most one thread at a time
for a given (PackFile,position) tuple.load() invocation there is exactly one
createRef(PackFile, long, ByteWindow) invocation to wrap a
SoftReference around the cached entity.createRef() there will be
exactly one call to clear(Ref) to cleanup any resources associated
with the (now expired) cached entity.
Therefore, it is safe to perform resource accounting increments during the
load(PackFile, long) or
createRef(PackFile, long, ByteWindow) methods, and matching
decrements during clear(Ref). Implementors may need to override
createRef(PackFile, long, ByteWindow) in order to embed additional
accounting information into an implementation specific Ref subclass,
as the cached entity may have already been evicted by the JRE's garbage
collector.
To maintain higher concurrency workloads, during eviction only one thread performs the eviction work, while other threads can continue to insert new objects in parallel. This means that the cache can be temporarily over limit, especially if the nominated eviction thread is being starved relative to the other threads.
| Modifier and Type | Method and Description |
|---|---|
static void |
reconfigure(WindowCacheConfig cfg)
Deprecated.
use
cfg.install() to avoid internal reference. |
@Deprecated public static void reconfigure(WindowCacheConfig cfg)
cfg.install() to avoid internal reference.The new configuration is applied immediately. If the new limits are smaller than what what is currently cached, older entries will be purged as soon as possible to allow the cache to meet the new limit.
cfg - the new window cache configuration.IllegalArgumentException - the cache configuration contains one or more invalid
settings, usually too low of a limit.Copyright © 2016 Eclipse JGit Project. All rights reserved.